1. Field of the Invention
The present invention relates to an image forming apparatus and a method for adjusting an image forming apparatus, for forming a multi-color image by superimposing color images of respective colors and performing a color registration adjustment to correct color misregistration of the multi-color image.
2. Description of Related Art
In an image forming apparatus for forming a multi-color image, after decomposing inputted data into respective color components, namely black (K), cyan (C), magenta (M) and yellow (Y), and performing image processing, image data of the respective color components are created, and a multi-color image is formed by superimposing color images of the respective colors based on the respective image data. When forming a multi-color image, if the positions of the respective color images superimposed deviate from each other, color misregistration occurs in the formed multi-color image and the image quality is lowered. In particular, in a so-called tandem type image forming apparatus comprising an image forming unit for each color component to improve the speed of forming a multi-color image, the respective color images are formed in the respective image forming units separately, and a multi-color image is formed by superimposing the respective color images one after another on a recording medium such as recording paper, or on a transfer medium used for transferring the multi-color image to a recording medium. Therefore, the positions on the transfer medium where the respective color images are formed easily deviate from each other, and consequently such an image forming apparatus has a serious problem that the image quality may be lowered by color misregistration.
Hence, in order to accurately superimpose the respective color images on the transfer medium, an image forming apparatus for forming a multi-color image performs a color registration adjustment for correcting color misregistration of a multi-color image. The conventional color registration adjustment is usually carried out by detecting a deviation of the image forming positions of other color images with respect to the image forming position of one color image to be a reference by an optical detector, determining a correction amount for the image forming position of each color image based on the detection results of the detector so that the image forming positions of the respective color images coincide with each other, and adjusting timings of forming the respective color images according to the determined correction amounts. With such conventional techniques, in order to detect a deviation between the image forming positions of respective color images, the respective color images are formed at predetermined timings, and the distance between the formed respective color images is detected, or the superimposed state and density of a multi-color image formed by superimposing the respective color images are measured.
For example, Japanese Patent Application Laid-Open No. 10-213940 (1998) discloses a technique of detecting a deviation between the positions of respective color images by measuring the distance between the positions on a transfer medium where the respective color images are formed, and correcting the positions of forming the respective color images on the transfer medium, based on the detected positional deviation. In this conventional technique, the distance between a color image to be a reference and other color images is detected with a detector, the amount of deviation of the position of each of the color images is determined based on the detected distance, and a deviation of the positions at which the respective color images are formed is corrected.
Further, Japanese Patent Application Laid-Open No. 2000-81744 discloses a technique in which the density of a multi-color image formed on a transfer medium by superimposing respective color images is measured, and a deviation of the positions at which the respective color images are formed is corrected so that the density of the multi-color image is equal to a density when the respective color images are accurately superimposed. With this conventional technique, in order to improve the correction accuracy, a plurality of line images of the same shape are formed, and the density of the line images formed in many colors is detected with a detector to find the superimposed state of the respective color images. Then, a state in which the detected density of the lines is within a predetermined density range is considered as a state in which the respective color images are accurately superimposed, and the position of forming each color image is corrected so that the respective color images are accurately superimposed.
By the way, in the conventional technique disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 10-213940 (1998), the deviation between the positions of the respective color images is found using the detector for detecting the positions on the medium where the respective color images are formed. In order to accurately detect the deviation between the positions of the respective color images, it is necessary to use a high-resolution detector, and consequently, the cost of the apparatus increases. In this structure, in order to decrease the cost, it is supposed to use a low-resolution detector by forming a plurality of color images of each color and finding an average value of the deviations of the respective color images. However, with a low-resolution detector, among reflection light to be incided on a light receiving unit in the light irradiated from the light emitting unit of the detector, the area of part of light reflected on the image is large, and consequently irregular reflection light also incides the light receiving unit in addition to regular reflection light, and a disturbance occurs in the output of the detector. The disturbance in the output of the detector caused by the irregular reflection light varies depending on various conditions such as the difference in the detection ability of the detector, the mount error of the detector, a temperature change in the image forming apparatus, and changes in the respective components with time, and thus it is difficult to deal with this problem and it is difficult to accurately detect the positional deviation of the respective color images.
On the other hand, in the conventional technique disclosed in Japanese Patent Application Laid-Open No. 2000-81744, for the entire region where a color registration adjustment is performed, it is necessary to find an adjustment amount for the position of each color image by forming test images, including test images in a state in which an image to be a reference and a color image to be subjected to positional adjustment are perfectly superimposed, while changing the superimposed state of the respective color images line by line. Thus, since it is necessary to form test images for color registration adjustment and detect the density for all regions capable of adjusting the positions of the respective color images, there is the problem that the time required for the color registration adjustment becomes longer. Moreover, a large amount of developer is used for the color registration adjustment, and the running cost increases. Further, if the time required for the color registration adjustment is shortened, or if the cost, more specifically the amount of developer necessary for the color registration adjustment is reduced, there arises the problem that the region capable of adjusting the positions of the respective color images becomes narrower.